Plasma technology has been pursued for treatment of liquids, such as e.g., water, for some time (Hoeben, 2000; Lee & Lee, 2003; Yamabe et al., 2004; Grabowski et al., 2004; Lambert & Kresnyak, 2000; Johnson, 1996, Johnson, 1997; Denes, 2004; Anpilov et al., 2004). The problem usually is to produce a homogeneous dielectric barrier discharge plasma with sufficient surface area in or above a liquid phase layer. The treatment that is usually associated with the generation of arcs, also called streamers, is referred to as Corona treatment rather than homogenous dielectric barrier discharge plasma treatment. Corona technology is often used in an air environment in combination with ozone or UV treatment in order to enhance the oxidative nature of the chemical reactions that take place during these processes. The generation of UV light, radicals, singlet oxygen, peroxides and oxidized species during these discharge processes is underlying the disinfection and purification of the liquid phase. However, to achieve sufficient mixing of these active species with the liquid phase that is to be treated is often a problem.
UV photo-catalysis is also used for disinfection and removal of micropollutants in liquids such as water. For this purpose, porous membranes or granulates can be loaded or coated with catalysts such as TiO2. Under the influence of UV or visible light, catalyzed oxidative reactions can take place on the surface of a carrier. The products of such reactions have a strong disinfecting potential.
Although some toxic organic compounds may be destroyed using either Corona treatment or UV photo-catalysis, a wide variety of residual micropollutant species cannot be eliminated using these techniques.
Most commonly, water is disinfected using chemical additives such as chlorine or biocides. Known drawbacks are that such agents often are hampered in their efficiency to kill non-bacterial species or cause the formation of undesired side products such as organic halogens subject to absorption (AOX) through interaction of e.g., chlorine with organic matter in water. Furthermore, chlorine and biocides have a negative impact on the quality of drinking water. Also some rest chemical oxygen demand (COD) can cause in certain niches post-growth of bacteria and may lead to infection and fouling of equipment and utilities.
A number of technical problems are identified regarding the use of submerged plasma technology aimed at disinfection and purification of liquids, such as e.g., water, and also gases, such as e.g., air. A first problem is how to generate a dielectric barrier discharge (DBD) plasma in a gaseous phase which is submerged into or surrounded by a liquid phase.
The geometry and positioning of the electrodes as well as the way and conditions in which both phases are mixed with one another are crucial to obtain a homogeneous dielectric barrier discharge plasma within the mixed phase.
The importance of using a homogeneous dielectric barrier discharge plasma rather than a Corona discharge plasma is manifest for the efficiency and efficacy of treatment, energy consumption and wear of the electrodes in the plasma reactor.
A second problem related to the use of plasma technology that is directed towards disinfection and purification of liquid or gaseous media is often posed in the requirement for industrial capacity. Using state-of-the art treatment equipment, practical limitations are often observed with flow rates of substrate liquid or gas streams. As a consequence, energy costs of operation and up scaling costs to meet capacity requirements may be high.
A problem associated with photo-catalyzed micropollutant removal processes is the degeneration of the catalyst that is used. This requires regeneration, or sometimes even replacement, of the catalyst involving downtime and extra costs for replacement of the catalyst. Documents U.S. Pat. Nos. 5,876,663 and 6,558,638 suffer from a number of the problems described above. In particular, the U.S. Pat. No. 6,558,638 reference describes a system wherein a plasma is produced in water. In this system, a tube is provided, produced from a dielectric material, and surrounded by a number of ring electrodes. This apparatus is submerged in the liquid to be treated, normally water. Air is pumped through the dielectric tube, and enters the water through apertures in the dielectric tube. The plasma discharge zone is present between the successive ring electrodes, i.e. plasma is created outside the tube volume, in the water and/or in the air bubbles entering the water. One electrode may have an elongate portion extending in the centre of the dielectric tube, but this is not an essential element: this central portion merely helps to decrease the capacitance of the first interelectrode gap (on the outside of the tube), and to thereby put a maximum portion of the voltage on said first gap, and then cause a sequence of successive breakdowns (‘slipping surface’ discharge). This technique has a number of drawbacks, the main one being a loss of power due to the existence of current in the water. This system also suffers from the fact that the flow of liquid through the apparatus is subjected to considerable flow restrictions, which puts a limit on the possible flow rates which can be processed. This system is also difficult to up-scale, due to its specific geometry, wherein the electrical field is coaxial to the flow direction of the treated liquid.